1. Field of the Invention
The present invention relates to a scanning optical device and an image forming apparatus using the same and, more particularly, to a device suitable for an apparatus such as a laser beam printer, digital copying machine, or the like, which has, e.g., an electrophotography process for recording image information by reflecting and deflecting (deflecting and scanning) at least one light beam, which has been optically modulated and emitted by light source means, by deflection means comprising a rotary polygonal mirror or the like, and scanning a surface to be scanned with the light beam via image forming means having at least one diffraction surface.
2. Related Background Art
Conventionally, in a scanning optical device of, e.g., a laser beam printer (LBP) or the like, a light beam, which has been optically modulated and emitted by light source means in accordance with an image signal, is periodically deflected by a beam deflector comprising, e.g., a rotary polygonal mirror, and is focused by image forming means having f-xcex8 characteristics to form a spot on the surface of a photosensitive recording medium (photosensitive drum), and to scan that surface with the beam spot, thus recording an image.
Furthermore, various scanning optical devices having diffraction surfaces in a portion of image forming means (scanning optical means) have been proposed in, e.g., Japanese Patent Application Laid-Open No. 10-68903 and the like. In Japanese Patent Application Laid-Open No. 10-68903, image forming means uses an optical element having a refraction portion (refraction surface) and diffraction portion (diffraction surface). When the powers of the refraction and diffraction portions are set to satisfy a desired condition, changes in magnification and focus in the main scanning direction due to the temperature drift of the scanning optical device are corrected by a change in power of the refraction and diffraction portions of the image forming means, and the wavelength drift of a semiconductor laser as light source means. With this arrangement, even when the temperature drifts, a high-quality image can be obtained.
The diffraction surface of the optical element which serves as the image forming means and has the refraction and diffraction surfaces is normally formed to have a grating pattern so that first-order diffraction light as diffraction light of an order to be used (use diffraction light) has a maximum intensity. At this time, of the diffraction light components diffracted by the diffraction surface, the amount of unnecessary (high-order) diffraction light components is smaller than the diffraction light of the order used to form a spot on the surface to be scanned. However, in a scanning optical device in which the angle of incidence onto the diffraction surface changes depending on the image height, the unnecessary (high-order) diffraction light components increase/decrease in correspondence with the image height. In the actual manufacturing process, since a manufacturing error is produced with respect to an ideal diffraction grating pattern, unnecessary (high-order) diffraction light components often increase.
Such unnecessary (high-order) diffraction light becomes flare as stray light, and adversely influences image quality.
Furthermore, a scanning lens system (image forming means) of a scanning optical device including such diffraction optical element is normally made up of a plastic lens, and an anti-reflection coat to be formed on the refraction surface is often omitted since it suffers many technical and cost problems. For this reason, reflected diffraction light produced by the diffraction surface is reflected by the refraction surface of the plastic lens without any anti-reflection coat, and becomes ghost as stray light.
This state will be explained below using FIGS. 10 and 11.
FIG. 10 is a sectional view showing principal part of the conventional scanning optical device in the main scanning direction.
In FIG. 10, a divergent light beam emitted by a light source means 91 is converted into a nearly collimated light beam by a collimator lens 92, and is limited by a stop 93. The light beam then enters a cylindrical lens 94 having a predetermined refracting power in only the sub scanning direction. Of the nearly collimated light beam that has entered the cylindrical lens 94, light components in the main scanning section leave the lens as they are. On the other hand, light components in the sub scanning section are focused, and form a nearly linear image on a deflection surface (reflection surface) 95a of a beam deflector 95 comprising a polygonal mirror.
A light beam 15 (15P, 15U, 15L) reflected and deflected by the beam deflector enters an image forming means (scanning lens system) 85 which comprises a refraction optical element 81 and diffraction optical element 82. In FIG. 10, a plastic toric lens 81 and long diffraction optical element 82 are inserted in turn from the side of the beam deflector 95. The long diffraction optical element 82 has different powers in the main scanning direction and sub scanning direction, forms image of the light beam coming from the beam deflector 95 on a surface 96 to be scanned, and corrects any inclination of the deflection surface (mirror surface) of the beam deflector 95. The light beam which has left the image forming means 85 forms an image on the surface 96 to be scanned, and optically scans the surface 96 to be scanned in the direction of an arrow B (main scanning direction) upon rotating the beam deflector 95 in the direction of an arrow A, thus recording image information.
In FIG. 10, the long diffraction optical element 82 has an entrance surface 83 serving as a refraction surface, and an exit surface 84 serving as a diffraction surface (diffraction grating surface). Most light components of the light beam 15 (15P, 15U, 15L) reflected and deflected by the beam deflector 95 are imaged on the surface 96 to be scanned as use diffraction light (normally, +1st-order diffraction light), thus forming a beam spot (not shown).
However, some light components of the light beam 15 (15P, 15U, 15L) reflected and deflected by the beam deflector 95 become unnecessary high-order diffraction light. Of these light components, sixth-order reflected diffraction light (reflected sixth-order diffraction light) diffracted by the diffraction surface 84 will be examined below.
In FIG. 10, of the reflected sixth-order diffraction light, a light beam 16 (16P, 16U, 16L) is surface-reflected by the refraction surface 83, is also diffracted by the diffraction surface 84, and travels toward the surface 96 to be scanned as use diffraction light (normally, +1st-order diffraction light). As can be seen from FIG. 10, such reflected sixth-order diffraction light hits the surface 96 to be scanned as stray light although it does not form any image.
The behavior of stray light of the reflected sixth-order diffraction light that scans the surface to be scanned will be explained below using FIG. 11. In FIG. 11, the abscissa plots the image height of a primary beam spot which reaches the surface 96 to be scanned, and the ordinate plots the position of stray light of the reflected sixth-order diffraction light on the surface 96 to be scanned. When the primary beam spot scans the surface 96 to be scanned, the stray light of the reflected sixth-order diffraction light scans the surface 96 to be scanned accordingly, and the scan speed lowers at image height positions near xc2x180 mm. As a result, many stray light components gather around the image height positions of xc2x180 mm, thus considerably deteriorating image quality.
Stray light such as flare, ghost, or the like blurs an image on the surface to be scanned. For example, in a laser beam printer (LBP), a blurred image is printed. Furthermore, in recent years, since the sensitivity of a photosensitive drum is increasing to express a halftone image, deterioration of image quality due to stray light is not negligible.
In one aspect of the invention, there is provided a scanning optical apparatus comprising incident optical means for causing at least one light beam emitted by light source means to be incident on deflection means, and image forming means, having at least one diffraction surface, for forming the image of the at least one light beam reflected and deflected by the deflection means on a surface to be scanned,
wherein the scanning optical apparatus further comprises limiting means, inserted in an optical path between said diffraction surface of said image forming means and the surface to be scanned, for limiting unnecessary diffraction light of another order with respect to diffraction light of an order used to form a spot on the surface to be scanned from among the diffraction light diffracted by said diffraction surface of said image forming means.
In further aspect of the scanning optical apparatus according to the invention, said limiting means is formed by substantially parallel edge portions which extend in a main scanning direction.
In further aspect of the scanning optical apparatus according to the invention, said limiting means is formed by substantially parallel slit members which extend in a main scanning direction.
In further aspect of the scanning optical apparatus according to the invention, said limiting means is formed by a reflection member for changing a direction in which beam of the diffraction light of the order used travels.
In further aspect of the scanning optical apparatus according to the invention, said limiting means is formed by an entrance window and/or an exit window formed on a casing which holds at least one of a plurality of components that build said scanning optical device.
In further aspect of the scanning optical apparatus according to the invention, a scan width of the unnecessary diffraction light satisfies:
Lm/Lo less than 0.8
where
Lm: the scan width of the unnecessary diffraction light
Lo: the effective scan width
In further aspect of the scanning optical apparatus according to the invention, a condition:
4xe2x89xa6m/nxe2x89xa67
is satisfied, where n represents the order used, and m represents the other order.
In further aspect of the scanning optical apparatus according to the invention, the diffraction light of the order used is first-order diffraction light, and the unnecessary diffraction light is sixth-order reflected diffraction light.
In further aspect of the scanning optical apparatus according to the invention, a condition:
xcfx86o less than S less than xcfx86m
is satisfied, where S represents the width of an aperture of said limiting means in the sub scanning direction, xcfx86o represents the light beam size with respect to the sub scanning direction of the light beam that is to form a primary beam spot, at the position of said limiting means, and xcfx86m represents the size of stray light of reflected sixth-order diffraction light with respect to the sub scanning direction.
In further aspect of the scanning optical apparatus according to the invention, a condition:
{square root over ((S/xcfx86m))} less than Lm/Lo
is satisfied, where S represents the width of an aperture of said limiting means in the sub scanning direction, xcfx86m represents the size of stray light of reflected sixth-order diffraction light with respect to the sub scanning direction, Lm represents the scan width of the unnecessary diffraction light, and Lo represents the effective scan width.
In another aspect of the invention, there is provided an image forming apparatus comprising the above scanning optical apparatus, a photosensitive member located at the surface to be scanned of said scanning optical apparatus, developing means for developing as a toner image an electrostatic latent image formed by scanning a surface of said photosensitive member with a light beam, transfer means for transferring the developed toner image onto a paper sheet, and fixing means for fixing the transferred toner image on the paper sheet.
In another aspect of the invention, there is provided an image forming apparatus comprising the above scanning optical apparatus, and a printer controller for converting code data input from an external device into an image signal, and inputting the image signal to said scanning optical apparatus.
It is an object of the present invention to provide a scanning optical apparatus in which limiting means for limiting unnecessary diffraction light produced by a diffraction surface is inserted in the optical path between the diffraction surface and a surface to be scanned so as to reduce unnecessary diffraction light that reaches the surface to be scanned, and an image forming apparatus using the same.
It is another object of the present invention to provide a scanning optical apparatus which improves the shape, size, and the like of members in the apparatus to reduce unnecessary diffraction light that reaches the surface to be scanned without adding any new limiting member.